Methods and devices for processing polymer films

ABSTRACT

Stretched polymeric films can be used in a variety of applications, including optical applications. The stretching conditions and shape of the stretching tracks in a stretching apparatus can determine or influence film properties. Take-away systems can be used to receive the film after stretching. The configuration of the take-away system can, in at least some instances, influence final film properties.

FIELD OF THE INVENTION

[0001] Generally, the present invention relates to methods and devicesfor stretching polymer films and the films obtained by the methods anddevices. The present invention also relates to methods and devices forstretching polymer films using take-away systems to receive the polymerfilm after stretching.

BACKGROUND OF THE INVENTION

[0002] There are a variety of reasons to stretch polymer films.Stretching can enhance or generate desired mechanical, optical, andother film properties. For example, polymer films can be stretched toprovide a desired degree of uniaxial or near uniaxial orientation inoptical properties. In general, perfect uniaxial orientation of abirefringent polymer results in a film (or layers of a film) in whichthe index of refraction in two of three orthogonal directions is thesame (for example, the width (W) and thickness (T) direction of a film,as illustrated in FIG. 4). The index of refraction in the thirddirection (for example, along the length (L) direction of the film) isdifferent from the indices of refraction in the other two directions.Typically, perfect uniaxial orientation is not required and some degreeof deviation from the optimal conditions can be allowed depending on avariety of factors including the end-use application of the polymerfilm.

[0003] In optical applications, a uniaxially oriented film can provideuseful optical properties such as more uniform performance across avariety of different viewing angles. Other applications can also benefitfrom uniaxial or near uniaxial orientation of a polymer film. Forexample, uniaxially oriented films are more easily fibrillated or tornalong the orientation direction.

SUMMARY OF THE INVENTION

[0004] Generally, the present invention relates to methods and devicesfor processing polymer films. One embodiment is an apparatus forstretching a film. The apparatus includes a conveyor and an isolatedtake-away system. The conveyor is configured and arranged to convey thefilm along a machine direction within the apparatus. The conveyorincludes gripping members that are configured and arranged to holdopposing edge portions of the film. A portion of the conveyor isconfigured and arranged to provide diverging paths along which thegripping members move to stretch the film. The isolated take-away systemreceives the film from the conveyor after stretching the film. Thetake-away system includes opposing tracks and gripping membersconfigured and arranged to grasp opposing take-away regions of the filmafter a desired amount of stretching and convey the opposing take-awayregions of the film along the opposing tracks. The opposing tracksdefine a region in which at least a portion of the opposing tracks areangled toward each other.

[0005] Another embodiment is a method of processing a film. The methodincludes holding opposing edge portions of a film using grippingmembers. The film is then conveyed along diverging paths and in amachine direction within a stretching region of a stretching apparatusto stretch the film. The film is then received, after stretching, in anisolated take-away system by grasping opposing take-away regions of thefilms using opposing gripping members of the take-away system. Withinthe take-away system, the film is conveyed through a portion of thetake-away system in which the opposing gripping members are angledtoward each other.

[0006] Yet another embodiment is an apparatus for processing a film. Theapparatus includes a conveyer and an isolated take-away system. Theconveyor has gripping members that hold opposing edge portions of thefilm and convey, under influence of a drive member, the film along amachine direction within apparatus. A portion of the conveyor isconfigured and arranged to provide diverging paths along which thegripping members move to stretch the film. The isolated take-away systemreceives the film from the conveyor after stretching the film. Thetake-away system includes a first set of opposing tracks, a second setof opposing tracks, a plurality of first gripping members configured andarranged to grasp opposing first take-away regions of the film after adesired amount of stretching and convey the film along the first setopposing tracks, and a plurality of second gripping members configuredand arranged to grasp opposing second take-away regions of the film andconvey the film along the second set of opposing tracks. The secondtake-away regions are disposed nearer a center of the film than thefirst take-away regions.

[0007] Another embodiment is an apparatus for processing a film. Theapparatus includes a conveyor and an isolated take-away system. Theconveyor has gripping members that hold opposing edge portions of thefilm and convey, under influence of a drive member, the film along amachine direction within the apparatus. A portion of the conveyor isconfigured and arranged to provide diverging paths along which thegripping members move to stretch the film. The isolated take-away systemreceives the film from the conveyor after stretching the film. Thetake-away system has opposing tracks and a plurality of gripping membersconfigured and arranged to grasp opposing take-away regions of the filmafter a desired amount of stretching and convey the opposing take-awayregions of the film along the opposing tracks. The opposing tracks aredisposed at an angle of at least 1° with respect to the machinedirection.

[0008] Another embodiment is an apparatus for processing a film. Theapparatus includes a conveyor and an isolated take-away system. Theconveyor has gripping members that hold opposing edge portions of thefilm and convey, under influence of a drive member, the film along amachine direction within the apparatus. A portion of the conveyor isconfigured and arranged to provide diverging paths along which thegripping members move to stretch the film. The isolated take-away systemreceives the film from the conveyor after stretching the film. Thetake-away system has opposing tracks and a plurality of gripping membersconfigured and arranged to grasp opposing take-away regions of the filmafter a desired amount of stretching and convey the opposing take-awayregions of the film along the opposing tracks. The apparatus isconfigured and arranged to allow selection of a final transversedirection draw ratio of the film by changing a position of the isolatedtake-away system with respect to a position of the conveyor.

[0009] Yet another embodiment is an apparatus for stretching a film. Theapparatus includes a conveyor and an isolated take-away system. Theconveyor is configured and arranged to convey the film along a machinedirection within the apparatus. The conveyor includes gripping membersthat are configured and arranged to hold opposing edge portions of thefilm. A portion of the conveyor is configured and arranged to providediverging paths along which the gripping members move to stretch thefilm. The isolated take-away system receives the film from the conveyorafter stretching the film. The take-away system includes opposing tracksand gripping members configured and arranged to grasp opposing take-awayregions of the film after a desired amount of stretching and convey theopposing take-away regions of the film along the opposing tracks. Theopposing tracks define a region in which at least a portion of theopposing tracks are angled away from each other.

[0010] Another embodiment is an apparatus for processing a film. Theapparatus includes a conveyor, a stretching region, and apost-conditioning region. The conveyor is configured and arranged toconvey the film along a machine direction. The conveyor has grippingmembers that are configured and arranged to hold opposing edge portionsof the film. In the stretching region the gripping members areconfigured arranged to travel along diverging paths to stretch the film.The post-conditioning region is disposed after the stretching region andincludes at least one zone in which the gripping members are configuredand arranged to travel along converging paths.

[0011] Other embodiments include methods of processing a film using anyof the apparatuses described above.

[0012] The above summary of the present invention is not intended todescribe each disclosed embodiment or every implementation of thepresent invention. The Figures and the detailed description which followmore particularly exemplify these embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The invention may be more completely understood in considerationof the following detailed description of various embodiments of theinvention in connection with the accompanying drawings, in which:

[0014]FIG. 1 is a schematic top view of a prior art tenter apparatusused to stretch film;

[0015]FIG. 2 is a perspective view of a portion of film in the prior artprocess depicted in FIG. 1 both before and after the stretching process;

[0016]FIG. 3 is a block diagram showing steps according to one aspect ofthe present invention;

[0017]FIG. 4 is a perspective view of a portion of film in a uniaxialstretching process both before and after the stretching process;

[0018]FIG. 5 is a schematic illustration of one embodiment of thestretching process and one embodiment of a stretching apparatusaccording to the present invention;

[0019]FIG. 6 is a schematic top view of a portion of a stretchingapparatus according to the present invention;

[0020]FIG. 7 is an end view of the apparatus of FIG. 6;

[0021]FIG. 8 is a schematic illustration of a portion of the tracks of astretching apparatus illustrating one embodiment of a pre-conditioningregion of the stretching apparatus according to the invention;

[0022]FIG. 9 is a schematic illustration of one embodiment of adjustabletracks for a primary stretching region of a stretching apparatusaccording to the invention;

[0023]FIG. 10 is a schematic illustration of one embodiment of atake-away system for a stretching apparatus according to the invention;

[0024]FIG. 11 is a schematic illustration of another embodiment of atake-away system for a stretching apparatus according to the invention;

[0025]FIG. 12 is a schematic illustration of a third embodiment of atake-away system for a stretching apparatus according to the invention;

[0026]FIG. 13 is a schematic illustration of a fourth embodiment of atake-away system for a stretching apparatus according to the invention;

[0027]FIG. 14 is a schematic illustration of a fifth embodiment of atake-away system for a stretching apparatus according to the invention;

[0028]FIG. 15 is a schematic illustration of another embodiment oftracks for a primary stretching region of a stretching apparatusaccording to the invention;

[0029]FIG. 16 is a schematic side cross-sectional view of one embodimentof tracks and a track shape control unit for a stretching apparatusaccording to the invention;

[0030]FIG. 17 is a schematic illustration of one embodiment of atake-away system, according to the invention, for using in, for example,a conventional stretching apparatus such as that illustrated in FIG. 1;

[0031]FIG. 18 is a graph of examples of suitable boundary trajectoriesfor a primary stretching region of a stretching apparatus according tothe invention;

[0032]FIG. 19 is a graph of examples of suitable boundary trajectoriesfor a primary stretching region of a stretching apparatus according tothe invention illustrating the use of different stretching regions withdifferent parabolic configurations;

[0033]FIG. 20 is a graph of examples of suitable boundary trajectoriesfor a primary stretching region of a stretching apparatus according tothe invention including boundary trajectories that are linearapproximations to suitable parabolic or substantially parabolic boundarytrajectories;

[0034]FIG. 21 is a schematic view of a portion of the track and trackshape control unit of one embodiment of FIG. 16; and

[0035]FIG. 22 is a schematic view of another portion of the track andtrack shape control unit of one embodiment of FIG. 16.

[0036] While the invention is amenable to various modifications andalternative forms, specifics thereof have been shown by way of examplein the drawings and will be described in detail. It should beunderstood, however, that the intention is not to limit the invention tothe particular embodiments described. On the contrary, the intention isto cover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0037] The present invention is believed to be applicable to methods anddevices for stretching polymer films and the films made using themethods and devices. In addition, the present invention is directed tomethods and devices for stretching polymer films using take-away systemsto receive the polymer film after stretching. The polymer films can bestretched using these methods and devices to achieve uniaxial or nearuniaxial orientation, if desired. The methods and devices can also beused to achieve other orientation conditions.

[0038] The present invention is applicable generally to a number ofdifferent polymer films, materials, and processes. The present inventionis believed to be particularly suited to the fabrication of polymeroptical films. The methods and devices can be used, if desired, to makeoptical films or other films having one or more properties selected fromimproved optical performance, improved optical properties, increasedpropensity to fracture or tear in a controlled manner or direction,enhanced dimensional stability, better processability, easiermanufacturability, and lower cost when compared to optical films madeusing conventional methods and devices.

[0039] A variety of optical films can be stretched or drawn according tothe present invention. The films can be single or multi-layer films:Suitable films are disclosed, for example, in U.S. Pat. Nos. 5,699,188;5,825,543; 5,882,574; 5,965,247; and 6,096,375; and PCT PatentApplications Publication Nos. WO 95/17303; WO 96/19347; WO 99/36812; andWO 99/36248 (the entire contents of each of which are hereinincorporated by reference). The devices and methods described hereininclude improvements, additions, or alterations to the devices andmethods described in U.S. patent application Ser. Nos. 10/156,347 and10/156,348 and U.S. Provisional Patent Application Serial No.60/294,490, all of which are incorporated herein by reference.

[0040] Films made in accordance with the present invention may be usefulfor a wide variety of products including, for example, polarizers,reflective polarizers, dichroic polarizers, aligned reflective/dichroicpolarizers, absorbing polarizers, and retarders (including z-axisretarders). The polymer films can be monolithic or multilayer polymerfilms. The polymeric films may also comprise layers of immiscible blendsthat form optical effects such as diffusers or diffuse reflectivepolarizers, such as described in U.S. Pat. Nos. 5,783,120; 5,825,543;5,867,316; 6,057,961; 6,111,696; and 6,179,948 and U.S. patentapplication Ser. Nos. 09/871,130 and 09/686,460, all of which areincorporated herein by reference. These polymer films can includecoatings or additional layers that are provided before or after drawing.Examples of some suitable coatings and layers are described in U.S. Pat.No. 6,368,699, incorporated herein by reference. In some embodiments,the polymer films include additional polarizing elements such as meltextrudable orienting dyes, wire grid polarizing elements, and the like.One example of a useful construction is a film with a layer of polyvinylalcohol (PVA) that is formed on the film, e.g. coated on the film priorto or after stretching the film. The PVA can be post-processed to form adichroic polarizing layer, e.g. through an iodine staining, aciddehydration or dye embedding methods. The substrate may itself be amonolithic film or a multilayer construction with or without opticalreflective power. Examples of PVA films suitable for use in thisconstruction can be found in U.S. Pat. No. 6,113,811, which isincorporated herein by reference.

[0041] One application of the particular films of the invention is as acomponent in devices such as, for example, polarizing beamsplitters forfront and rear projection systems or as a brightness enhancement filmused in a display (for example, a liquid crystal display) ormicrodisplay. It should also be noted that the stretcher described belowin accordance with the present invention may be used with a lengthorienter to make a mirror.

[0042] In general, the process includes stretching a film that can bedescribed with reference to three mutually orthogonal axes correspondingto the machine direction (MD), the transverse direction (TD), and thenormal direction (ND). These axes correspond to the width, length, andthickness of the film, as illustrated in FIG. 4. The stretching processstretches a region 20 of the film from an initial configuration 24 to afinal configuration 26. The machine direction is the general directionalong which the film travels through a stretching device, for example,the apparatus as illustrated in FIG. 5. The transverse direction is thesecond axis within the plane of the film and is orthogonal to themachine direction. The normal direction is orthogonal to both MD and TDand corresponds generally to the thickness dimension of the polymerfilm.

[0043]FIG. 3 is a block diagram of a process according to the presentinvention. In step 30, the film is supplied or provided to an apparatusfor stretching the film. The process optionally includes apreconditioning step 32. The film is stretched in step 34. The film isoptionally post-conditioned in step 36. The film is removed from thestretching apparatus in step 38.

[0044]FIG. 5 illustrates one embodiment of a stretching apparatus andmethod of the invention. It will be recognized that the processillustrated by FIG. 3 can be accomplished using one or more additionalapparatuses, apart from a stretching apparatus (which at a minimumperforms step 34 of FIG. 3). These one or more additional apparatusesperform one or more of the non-stretching functions (for example,functions represented by steps 30, 32, 36 and 38) illustrated in FIG. 3and shown in FIG. 5 as being performed by a stretching apparatus.

[0045] In the illustrated embodiment of FIG. 5, the apparatus includes aregion 30 where the film 40 is introduced into the stretching apparatus.The film can be provided by any desirable method. For example, the filmcan be produced in a roll or other form and then provided to stretchingapparatus. As another example, the stretching apparatus can beconfigured to receive the film from an extruder (if, for example, thefilm is generated by extrusion and ready for stretching after extrusion)or a coater (if, for example, the film is generated by coating or isready for stretching after receiving one or more coated layers) or alaminator (if, for example the film is generated by lamination or isready for stretching after receiving one or more laminated layers).

[0046] Generally, the film 40 is presented in region 30 to one or moregripping members that are configured and arranged to hold opposing edgesof the film and convey the film along opposing tracks 64 definingpredetermined paths. The gripping members 70 (see FIG. 7) typically holdthe film at or near the edges of the film. The portions of the film heldby the gripping members are often unsuitable for use after stretching sothe position of the gripping members is typically selected to providesufficient grip on the film to permit stretching while controlling theamount of waste material generated by the process.

[0047] One example of suitable gripping members includes a series ofclips that sequentially grip the film between opposing surfaces and thentravel around a track. The gripping members can nest or ride in a grooveor a channel along the track. Another example is a belt system thatholds the film between opposing belts or treads, or a series of belts ortreads, and directs the film along the track. Belts and treads can, ifdesired, provide a flexible and continuous, or semi-continuous, filmconveyance mechanism. A variety of opposing, multiple belt methods aredescribed, for example, in U.S. Pat. No. 5,517,737 or in European PatentApplication Publication No.0236171 μl (the entire contents of each ofwhich are herein incorporated by reference). The tension of belts isoptionally adjustable to obtain a desired level of gripping.

[0048] A belt or clip can be made of any material. For example, a beltcan be a composite construction. One example of a suitable belt includesan inner layer made of metal, such as steel, to support high tension andan outer layer of elastomer to provide good gripping. Other belts can beused. In some embodiments, the belt includes discontinuous tread toprovide good gripping.

[0049] Other methods of gripping and conveying the film through astretcher are known and may be used. In some embodiments, differentportions of the stretching apparatus can use different types of grippingmembers.

[0050] Gripping members, such as clips, can be directed along the trackby, for example, rollers 62 rotating a chain along the track with thegripping members coupled to the chain. The rollers are connected to adriver mechanism that controls the speed and direction of the film as itis conveyed through the stretching apparatus. Rollers can also be usedto rotate and control the speed of belt-type gripping members. The beltsand rollers optionally include interlocking teeth to reduce or preventslippage between the belt and roller.

[0051]FIGS. 6 and 7 illustrate one embodiment of the gripping membersand track. The gripping members 70 of this embodiment are a series oftenter clips. These clips can afford overall flexibility viasegmentation. The discrete clips are typically closely packed andattached to a flexible structure such as a chain. The flexible structurerides along or in channels along the track 64. Strategically placed camsand cam surfaces open and close the tenter clips at desired points. Theclip and chain assembly optionally ride on wheels or bearings or thelike. As one example, the gripping members are tenter clips mounted ontop and bottom bearings rolling between two pairs of inner and outerrails. These rails form, at least in part, the track.

[0052] The edges of the gripping members define a boundary edge for theportion of the film that will be stretched. The motion of the grippingmembers along the tracks provides a boundary trajectory that is, atleast in part, responsible for the motion and drawing of the film. Othereffects (e.g., downweb tension and take-up devices) may account forother portions of the motion and drawing. The boundary trajectory istypically more easily identified from the track or rail along which thegripping members travel. For example, the effective edge of the centerof the gripping member, e.g. a tenter clip, can be aligned to trace thesame path as a surface of the track or rail. This surface then coincideswith the boundary trajectory. In practice, the effective edge of thegripping members can be somewhat obscured by slight film slippage fromor flow out from under the gripping members, but these deviations can bemade small.

[0053] In addition, for gripping members such as tenter clips the lengthof the edge face can influence the actual boundary trajectory. Smallerclips will in general provide better approximations to the boundarytrajectories and smaller stretching fluctuations. In at least someembodiments, the length of a clip face edge is no more that one-half,and can be no more than one-quarter, the total initial distance betweenthe opposing boundary trajectories or tracks.

[0054] The two opposing tracks are optionally disposed on two separateor separable platforms or are otherwise configured to allow the distancebetween the opposing tracks to be adjustable. This can be particularlyuseful if different sizes of film are to be stretched by the apparatusor if there is a desire to vary the stretching configuration in theprimary stretching region, as discussed below. Separation or variationbetween the opposing tracks can be performed manually, mechanically (forexample, using a computer or other device to control a driver that canalter the separation distance between the tracks), or both.

[0055] Since the film is held by two sets of opposing gripping membersmounted on opposing tracks, there are two opposing boundarytrajectories. In at least some embodiments, these trajectories aremirror images about an MD center line of the drawing film. In otherembodiments, the opposing tracks are not mirror images. Such anon-mirror image arrangement can be useful in providing a variation (forexample, a gradient or rotation of principal axes) in one or moreoptical or physical properties across the film.

[0056] Returning to FIG. 5, the apparatus optionally includes apreconditioning region 32 that typically is enclosed by an oven 54 orother apparatus or arrangement to heat the film in preparation forstretching. The preconditioning region can include a preheating zone 42,a heat soak zone 44, or both. In at least some embodiments, there may bea small amount of film stretching that occurs in order to set thecontact between the gripping members and the film, as illustrated by theboundary trajectory of FIG. 8. In at least some instances, there may notactually be any stretching but the increase in separation between theopposing tracks may account, at least in part, for thermal expansion ofthe film as the film is heated.

[0057]FIG. 8 illustrates a supply region 30′ followed by thepreconditioning region 32′ and primary stretching region 34′. Within thepreconditioning region 32′ (or optionally in the supply region 30′) agripping member set zone 31′ is provided in which the tracks divergeslightly to set the gripping members (for example, tenter clips) on thefilm. The film is optionally heated within this zone. This initial TDstretch is typically no more than 5% of the final TD stretch andgenerally less than 2% of the final TD stretch and often less than 1% ofthe final TD stretch. In some embodiments, the zone in which thisinitial stretch occurs is followed by a zone 33′ in which the tracks aresubstantially parallel and the film is heated or maintained at anelevated temperature.

[0058] Returning to FIG. 5, the film is stretched in the primarystretching region 34. Typically, within the primary stretching region 34the film is heated or maintained in a heated environment above the glasstransition of the polymer(s) of the film. For polyesters, thetemperature range is typically between 80° C. and 160° C. Examples ofsuitable heating elements include convective and radiative heatingelements, although other heating elements can also be used. In someembodiments, the heating elements used to heat the film can becontrolled individually or in groups to provide a variable amount ofheat. Such control can be maintained by a variety of processes includingvariability in the temperature of the heating elements or in thedirection or speed of air directed from the heating element to the film.The control of the heating elements can be used, if desired, to variablyheat regions of the film to improve or otherwise alter uniformity ofstretching across the film. For example, areas of the film that do notstretch as much as other areas under uniform heating can be heated moreto allow easier stretching.

[0059] Within the primary stretching region 34, the gripping membersfollow generally diverging tracks to stretch the polymer film by adesired amount. The tracks in the primary stretching region and in otherregions of the apparatus can be formed using a variety of structures andmaterials. Outside of the primary stretching region, the tracks aretypically substantially linear. The opposing linear tracks can beparallel or can be arranged to be converging or diverging. Within theprimary stretching region, the tracks are generally diverging and aregenerally curvilinear, as described below.

[0060] In all regions of the stretching apparatus, the tracks can beformed using a series of linear or curvilinear segments that areoptionally coupled together. The tracks can be made using segments thatallow two or more (or even all) of the individual regions to beseparated (for example, for maintenance or construction). As analternative or in particular regions or groups of regions, the trackscan be formed as a single continuous construction. The tracks caninclude a continuous construction spanning one or more adjacent regionsof the stretcher. The tracks can be any combination of continuousconstructions and individual segments.

[0061] In at least some embodiments, the tracks in the primarystretching region are coupled to, but separable from, the tracks of thepreceding regions. The tracks 140, 141 in the succeedingpost-conditioning or removal regions are typically separated from thetracks of the primary stretching region, as illustrated, for example, inFIG. 5.

[0062] Although the tracks in the primary stretching region arecurvilinear, linear track segments can be used in at least someembodiments. These segments are aligned (by, for example, pivotingindividual linear segments about an axis) with respect to each other toproduce a linear approximation to a desired curvilinear trackconfiguration. Generally, the shorter the linear segments are, thebetter the curvilinear approximation can be made. In some embodiments,the positions of one or more, and preferably all, of the linear segmentsare adjustable (pivotable about an axis) so that the shape of the trackscan be adjusted if desired. Adjustment can be manual or the adjustmentcan be performed mechanically, preferably under control of a computer orother device coupled to a driver. It will be understood, thatcurvilinear segments can be used instead of or in addition to linearsegments.

[0063] Continuous tracks can also be used through each of the regions.In particular, a continuous, curvilinear track can be used through theprimary stretching region. The continuous, curvilinear track typicallyincludes at least one continuous rail that defines the track along whichthe gripping members run. In one embodiment, the curvilinear trackincludes two pairs of inner and outer rails with tenter clips mounted ontop and bottom bearings rolling between the four rails.

[0064] In some embodiments, the continuous track is adjustable. Onemethod of making an adjustable continuous track includes the use of oneor more track shape control units. These track shape control units arecoupled to a portion of the continuous track, such as the continuousrail, and are configured to apply a force to the track as required tobend the track. FIG. 9 schematically illustrates one embodiment of suchan arrangement with the track shape control units 65 coupled to thetrack 64. Generally, the track shape control units have a range offorces that the track shape control unit can apply, although someembodiments may be limited to control units that are either on or off.The track shape control units can typically apply a force toward thecenter of the film or apply a force away from the center of the film or,preferably, both. The track shape control units can be coupled to aparticular point on the adjustable continuous track or the track shapecontrol units can be configured so that the track can slide laterallyalong the control unit while still maintaining coupling between thetrack and control unit. This arrangement can facilitate a larger rangeof motion because it allows the track to more freely adjust as thecontrol units are activated. Generally, the track shape control unitsallow the track to move through a range of shapes, for example, shapes67 and 69 of FIG. 9. Typically, the track shape control unit and thetrack can move along a line (or other geometric shape) of motion. Whenmore than one track shape control unit is used, the track shape controlunits can have the same or similar lines of motion and ranges of motionor the lines and ranges of motions for the individual track shapecontrol units can be different.

[0065] One example of a suitable track shape control unit and track isillustrated in FIG. 16. The track in this embodiment includes four rails400 with tenter clips (not shown) mounted on bearings (not shown)rolling between the four rails. The track shape control unit includes abase 402 that is coupled to a driver (not shown), top and bottom innercontact members 404, and top and bottom outer contact members 406. Theinner and outer contact members 404, 406 are coupled to the base 402 sothat moving the base allows the contact members to apply a force toinner and outer surfaces of the rails, respectively. Preferably, theinner and outer contact members have a shape, when viewed from above orbelow, that provides only small areas of contact between the innercontact members 406 an the rails 400, as illustrated in FIG. 21 (whichonly: shows the rails 400 and inner contact member 406). Examples ofsuch shapes include circular and ovoid, as well as diamond, hexagonal,or other similar shapes where contact between the inner contact members406 and the rails is made at the apex of these shapes. The outer contactmembers 404 can be similarly fashioned so that the portion of the outercontact member, when viewed from above or below, comes to a point tomake contact with the rails 400, as illustrated in FIG. 22 (which onlyshows the rails 400 and the portion of the outer contact member 404 thatmakes contact with the rails). Using such shapes allows the track shapecontrol unit to exert a force, if desired, to modify the track shapewhile allowing the track to slide laterally through the control unitrather than being fixed to the control unit. This configuration can alsoallow the track to adjust its instantaneous slope within the controlunit. For one or both of these reasons, the track can have a largerrange of shape adjustment. In other embodiments, there can be fewer ormore contact members or there may be only inner or only outer contactmembers.

[0066] Returning to FIG. 9, in some embodiments, one or more points 73of the track are fixed. The fixed points can be anywhere along the trackincluding at or near the start (as illustrated in FIG. 9) or end of theprimary stretching region. The fixed points 73 can also be positioned atother points along the track as illustrated in FIG. 15.

[0067] As further illustrated in FIG. 15, the tracks can be configuredto provide zones 81, 83, 85 within the primary stretching region thathave different stretching characteristics or that can be described bydifferent mathematical equations. In some embodiments, the tracks have ashape that defines these different zones. In other embodiments thetracks can be adjusted, using for example the track shape control unitsdiscussed above, to provide a variety of shapes 87, 89 beyond simple,monofunctional arrangements. This can be advantageous because it allowsdifferent portions of the primary stretching region to accomplishdesired functions. For example, an initial stretching zone may have aparticular shape (for example, a super-uniaxial shape with U>1 and F>1as described below) followed by one or more later zones with differentshapes (for example, a uniaxial shape). Optionally, intermediate zonescan be provided that transition from one shape to another. In someembodiments, the individual zones can be separated or defined by points73 of the track that are fixed.

[0068] In some embodiments, the track has a non-uniform cross-sectionalshape along the length of the track to facilitate bending and shaping ofthe track. For example, one or more rails used in the track can havedifferent cross-sectional shapes. As an example, in the four railconstruction described above each of the rails, or a subset of therails, has a varied cross-section along the length of the track. Thecross-section can be varied by, for example, altering either the heightor thickness of the track (or a component of the track such as one ormore continuous rails) or both. As an example, in one embodiment thethickness of the track or one or more rails in the track decreases orincreases along the length of the track in the machine direction. Thesevariations can be used to support a particular track shape or avariation in track shape adjustability. For example, as described abovethe track may have several different zones, each zone having a differenttrack shape. The cross-sectional variation of the track or component ofthe track can vary within each zone to achieve or facilitate aparticular rail shape and can vary between zones. As an example, a zonewith a relatively thick cross-sectional shape can be disposed betweentwo other zones to isolate or provide a transitional space between thetwo zones.

[0069] As an example of variation in track or rail cross-section, thearclength, s, can be used to represent a position along the track in thedesign of the thickness profile of a track or portion of a track, suchas a rail. The arclength, s, at the start of draw is defined as zero andat the other end of the draw is defined as L with correspondingthicknesses at the beginning and end of draw being designated as h(0)and h(L), respectively. The track or track component (e.g., rail) inthis particular embodiment has a taper over a portion of the beam fromL′ to L″ between s=0 and s=L such that the thickness h(L′) at positionL′ is greater than the thickness h(L″) at position L″. In this manner,either L′ or L″ may be at the higher arclength coordinate (i.e., L′>L″or L′<L″). One example of a useful thickness profile is a taper given bythe function for thickness, h(s), as a function of arclength s over therail from L′ to L″ is provided by the equation:

h(s)=(h(L′)−h(L″))(1−(s−L′)/(L″−L′))^(α) +h(L″)

[0070] where α is the positive rate of taper resulting in decreasingthickness from L′ to L″. When L′ is less than L″ this results in adecreasing thickness with arclength. When L′ is greater than L″ thisresults in a increasing thickness with arclength. The track canoptionally be apportioned into sections, each with its own local L′, L″and rate of taper. The maximum thickness of the track or track componentdepends on the amount of flexibility desired at that point on the track.Using beam theory, it can be shown that in the case of a straight beamwith a taper, a value for α of one third provides a beam that bendsparabolically in response to a load at one end. When the beam begins ina curved equilibrium configuration or is loaded by several controlpoints, other tapers may be more desirable. For transformation across avariety of other shapes, it may be useful to have both increasing anddecreasing thickness within a given track or track component, ornumerically calculated forms of the taper over any of these sections.The minimum thickness at any point along the track or track componentdepends on the amount of required strength of the track to support thedrawing forces. The maximum thickness can be a function of the level ofneeded flexibility. It is typically beneficial to maintain the level oftrack adjustment within the elastic range of the track or trackcomponent, e.g. to avoid the permanent yielding of the track or trackcomponent and loss of repeatable adjustment capability.

[0071] The paths defined by the opposing tracks affect the stretching ofthe film in the MD, TD, and ND directions. The stretching (or drawing)transformation can be described as a set of draw ratios: the machinedirection draw ratio (MDDR), the transverse direction draw ratio (TDDR),and the normal direction draw ratio (NDDR). When determined with respectto the film, the particular draw ratio is generally defined as the ratioof the current size (for example, length, width, or thickness) of thefilm in a desired direction (for example, TD, MD, or ND) and the initialsize (for example, length, width, or thickness) of the film in that samedirection. Although these draw ratios can be determined by observationof the polymer film as drawn, unless otherwise indicated reference toMDDR, TDDR, and NDDR refers to the draw ratio determined by a track usedto stretch the polymer film.

[0072] At any given point in the stretching process, TDDR corresponds toa ratio of the current separation distance of the boundary trajectories,L, and the initial separation distance of the boundary trajectories, L₀,at the start of the stretch. In other words, TDDR=L/L₀. In someinstances (as in FIGS. 2 and 4), TDDR is represented by the symbol λ. Atany given point in the stretching process, MDDR is the cosine of thedivergence angle, θ, the positive included angle between MD and theinstantaneous tangent of the boundary trajectory, e.g. track or rail. Itfollows that cot(θ) is equal to the instantaneous slope (i.e., firstderivative) of the track at that point. Upon determination of TDDR andMDDR, NDDR=1/(TDDR*MDDR) provided that the density of the polymer filmis constant during the stretching process. If, however, the density ofthe film changes by a factor of ρ_(f), where ρ_(f)=ρ/ρ₀ with ρ being thedensity at the present point in the stretching process and ρ₀ being theinitial density at the start of the stretch, then NDDR=ρ_(f)/(TDDR*MDDR)as expected. A change in density of the material can occur for a varietyof reasons including, for example, due to a phase change, such ascrystallization or partial crystallization, caused by stretching orother processing conditions.

[0073] Perfect uniaxial drawing conditions, with an increase indimension in the transverse direction, result in TDDR, MDDR, and NDDR ofλ, (λ)^(−1/2), and (λ)^(−1/2), respectively, as illustrated in FIG. 2(assuming constant density of the material). In other words, assuminguniform density during the draw, a uniaxially oriented film is one inwhich MDDR=(TDDR)^(−1/2) throughout the draw. A useful measure of theextent of uniaxial character, U, can be defined as:$U = \frac{\frac{1}{MDDR} - 1}{{TDDR}^{1/2} - 1}$

[0074] For a perfect uniaxial draw, U is one throughout the draw. When Uis less than one, the drawing condition is considered “subuniaxial”.When U is greater than one, the drawing condition is considered“super-uniaxial”. In a conventional tenter, where the polymer film isstretched linearly along tracks 2, as illustrated in FIGS. 1 and 2, tostretch a region 4 of the film to a stretched region 6 and thedivergence angle is relatively small (e.g., about 3° or less), MDDR isapproximately 1 and U is approximately zero. If the film is biaxiallydrawn so that MDDR is greater than unity, U becomes negative. In someembodiments, U can have a value greater than one. States of U greaterthan unity represent various levels of over-relaxing. These over-relaxedstates produce an MD compression from the boundary edge. If the level ofMD compression is sufficient for the geometry and material stiffness,the film will buckle or wrinkle.

[0075] As expected, U can be corrected for changes in density to giveU_(f) according to the following formula:$U_{f} = \frac{\frac{1}{MDDR} - 1}{\left( \frac{TDDR}{\rho_{f}} \right)^{1/2} - 1}$

[0076] Preferably, the film is drawn in plane (i.e., the boundarytrajectories and tracks are coplanar) such as shown in FIG. 5, althoughnon-coplanar stretching trajectories are also acceptable. The design ofin-plane boundary trajectories is simplified because the in-planeconstraint reduces the number of variables. The result for a perfectuniaxial orientation is a pair of mirror symmetric, in-plane, parabolictrajectories diverging away from the in-plane MD centerline. Theparabola may be portrayed by first defining TD as the “x” direction andMD as the “y” direction. The MD centerline between the opposing boundingparabolas may be taken as the y coordinate axis. The coordinate originmay be chosen to be the beginning of the primary stretching region andcorresponds to the initial centerpoint of the central trace between theparabolic trajectories. The left and right bounding parabolas are chosento start (y=0) at minus and plus x₀, respectively. The right boundingparabolic trajectory, for positive y values, that embodies thisembodiment of the invention is:

x/x ₀=(1/4)(y/x ₀)²+1

[0077] The left bounding parabolic trajectory is obtained by multiplyingthe left-hand side of the above equation by minus unity. In thediscussion below, descriptions of and methods for determining the rightbounded trajectory are presented. A left bounded trajectory can then beobtained by taking a mirror image of the right bounded trajectory overthe centerline of the film.

[0078] The coplanar parabolic trajectory can provide uniaxialorientation under ideal conditions. However, other factors can affectthe ability to achieve uniaxial orientation including, for example,non-uniform thickness of the polymer film, non-uniform heating of thepolymer film during stretching, and the application of additionaltension (for example, machine direction tension) from, for example,down-web regions of the apparatus. In addition, in many instances it isnot necessary to achieve perfect uniaxial orientation. Instead, aminimum or threshold U value or an average U value that is maintainedthroughout the draw or during a particular portion of the draw can bedefined. For example, an acceptable minimum/threshold or average U valuecan be 0.7, 0.75, 0.8, 0.85, 0.9, or 0.95, as desired, or as needed fora particular application.

[0079] As an example of acceptable nearly uniaxial applications, theoff-angle characteristics of reflective polarizers used in liquidcrystalline display applications is strongly impacted by the differencein the MD and ND indices of refraction when TD is the principalmono-axial draw direction. An index difference in MD and ND of 0.08 isacceptable in some applications. A difference of 0.04 is acceptable inothers. In more stringent applications, a difference of 0.02 or less ispreferred. For example, the extent of uniaxial character of 0.85 issufficient in many cases to provide an index of refraction differencebetween the MD and ND directions in polyester systems containingpolyethylene naphthalate (PEN) or copolymers of PEN of 0.02 or less at633 nm for mono-axially transverse drawn films. For some polyestersystems, such as polyethylene terephthalate (PET), a lower U value of0.80 or even 0.75 may be acceptable because of lower intrinsicdifferences in refractive indices in non-substantially uniaxially drawnfilms.

[0080] For sub-uniaxial draws, the final extent of truly uniaxialcharacter can be used to estimate the level of refractive index matchingbetween the y (MD) and z (ND) directions by the equation

Δn _(yz) =Δn _(yz)(U=0)×(1−U)

[0081] where Δn_(yz) is the difference between the refractive index inthe MD direction (i.e., y-direction) and the ND direction (i.e.,z-direction) for a value U and Δn_(yz)(U=0) is that refractive indexdifference in a film drawn identically except that MDDR is held at unitythroughout the draw. This relationship has been found to be reasonablypredictive for polyester systems (including PEN, PET, and copolymers ofPEN or PET) used in a variety of optical films. In these polyestersystems, Δn_(yz)(U=0) is typically about one-half or more the differenceΔn_(xy)(U=0) which is the refractive difference between the two in-planedirections MD (y-axis) and TD (x-axis). Typical values for Δn_(xy)(U=0)range up to about 0.26 at 633 nm. Typical values for Δn_(yz)(U=0) rangeup to 0.15 at 633 nm. For example, a 90/10 coPEN, i.e. a copolyestercomprising about 90% PEN-like repeat units and 10% PET-like repeatunits, has a typical value at high extension of about 0.14 at 633 nm.Films comprising this 90/10 coPEN with values of U of 0.75, 0.88 and0.97 as measured by actual film draw ratios with corresponding values ofΔn_(yz) of 0.02, 0.01 and 0.003 at 633 nm have been made according tothe methods of the present invention.

[0082] One set of acceptable parabolic trajectories that are nearly orsubstantially uniaxial character can be determined by the followingmethod. This described method determines the “right” boundary trajectorydirectly, and the “left” boundary trajectory is taken as a mirror image.First, a condition is set by defining an instantaneous functionalrelationship between TDDR measured between the opposing boundarytrajectories and MDDR defined as the cosine of the non-negativedivergence angle of those boundary trajectories, over a chosen range ofTDDR. Next, the geometry of the problem is defined as described in thediscussion of the parabolic trajectories. x ₁ is defined as the initialhalf distance between the boundary trajectories and a ratio (x/x₁) isidentified as the instantaneous TDDR, where x is the current x positionof a point on the boundary trajectory. Next, the instantaneousfunctional relationship between the TDDR and MDDR is converted to arelationship between TDDR and the divergence angle. When a specificvalue of U is chosen, the equations above provide a specificrelationship between MDDR and TDDR which can then be used in thealgorithm to specify the broader class of boundary trajectories thatalso includes the parabolic trajectories as a limiting case when Uapproaches unity. Next, the boundary trajectory is constrained tosatisfy the following differential equation:

d(x/x ₁)/d(y/x ₁)=tan(θ)

[0083] where tan(θ) is the tangent of the divergence angle θ, and y isthe y coordinate of the current position of the opposing point on theright boundary trajectory corresponding to the given x coordinate. Next,the differential equation may be solved, e.g. by integrating 1/tan(θ)along the history of TDDR from unity to the maximum desired value toobtain the complete coordinate set {(x,y)} of the right boundarytrajectory, either analytically or numerically.

[0084] As another example of acceptable trajectories, a class ofin-plane trajectories can be described in which the parabolic trajectoryis used with smaller or larger initial effective web TD length. If x₁ ishalf of the separation distance between the two opposing boundarytrajectories at the inlet to the primary stretching region (i.e. theinitial film TD dimension minus the selvages held by the grippers whichis the initial half distance between opposing boundary trajectories),then this class of trajectories is described by the following equation:

±(x)/(x ₁)=(1/4)(x ₁ /x ₀)(y/x ₁)²+1

[0085] where x₁/x₀ is defined as a scaled inlet separation. The quantityx₀ corresponds to half of the separation distance between two opposingtracks required if the equation above described a parabolic tracks thatprovided a perfectly uniaxial draw. The scaled inlet separation, x₁/x₀,is an indication of the deviation of the trajectory from the uniaxialcondition. In one embodiment, the distance between the two opposingtracks in the primary stretching zone is adjustable, as described above,allowing for the manipulation of the trajectory to provide values of Uand F different than unity. Other methods of forming these trajectoriescan also be used including, for example, manipulating the shape of thetrajectories using track shape control units or by selecting a fixedshape that has the desired trajectory.

[0086] For super-uniaxial draws, the severity of the wrinkling can bequantified using the concept of overfeed. The overfeed, F, can bedefined as the uniaxial MDDR (which equals (TDDR)^(−1/2)) divided by theactual MDDR. If the actual MDDR is less than the uniaxial MDDR, theoverfeed F is less than unity and the MDDR is under-relaxed resulting ina U less than unity. If F is greater than unity, the draw issuper-uniaxial and the MDDR is over-relaxed relative to the uniaxialcase. At least a portion of the extra slack can be accommodated as awrinkle because the compressive buckling threshold is typically low forthin, compliant films. When F is greater than unity, the overfeedcorresponds at least approximately to the ratio of the actual filmcontour length in the wrinkles along MD to the in-plane contour lengthor space.

[0087] Because of the relationship between TDDR and MDDR in the case ofconstant density, F can be written as:

F=1/(MDDR×TDDR ^(1/2))

[0088] Typically, F is taken as density independent for design purposes.Large values of F anytime during the process can cause large wrinklesthat can fold over and stick to other parts of the film causing defects.In at least some embodiments, the overfeed, F, remains at 2 or lessduring the draw to avoid or reduce severe wrinkling or fold over. Insome embodiments, the overfeed is 1.5 or less throughout the course ofthe draw. For some films, a maximum value of F of 1.2 or even 1.1 isallowed throughout the draw.

[0089] For at least some embodiments, particularly embodiments with U>1through the entire draw, rearranging the definition of overfeed providesa relative bound on a minimum MDDR given a current TDDR:

MDDR>1/(F _(max) ×TDDR ^(1/2))

[0090] where F_(max) can be chosen at any preferred level greater thanunity. For example, F can be selected to be 2, 1.5, 1.2, or 1.1, asdescribed above.

[0091] When the over-feed is less than unity, there is effectively morein-plane space along MD than is desired for the truly uniaxial draw andthe MDDR may be under-relaxed and causing MD tension. The result can bea U value less than unity. Using the relationships between U, F, MDDRand TDDR there is a corresponding correlation between U and F whichvaries with TDDR. At a critical draw ratio of 2, a minimum U valuecorresponds to a minimum overfeed of about 0.9. For at least someboundary trajectories including boundary trajectories in which U>1 forthe entire draw, MDDR can be selected to remain below a certain levelduring a final portion of draw, e.g.

MDDR<1/(F _(min) ×TDDR ^(1/2))

[0092] where F_(min) is 0.9 or more for a final portion of draw after adraw ratio of 2.

[0093] As an example, trajectories can be used in whichMDDR<(TDDR)^(−1/2) (i.e., U>1) throughout the stretch, F_(max) is 2, andthe film is stretched to a TDDR of at 4. If the trajectories arecoplanar, then the film is stretched to a TDDR of at least 2.4 and oftenat least 5.3. If F_(max) is 1.5, then the film is stretched to a TDDR ofat least 6.8. If the trajectories are coplanar, then the film isstretched to a TDDR of at least 2.1 and often at least 4.7, If F_(max)is 1.2, then the film is stretched using coplanar trajectories to a TDDRof at least 1.8 and often at least 4.0. For coplanar or non-coplanarboundary trajectories, if no limit is placed on F, then the film isstretched to a TDDR of greater than 4 and often of at least 6.8.

[0094] In another example, coplanar trajectories can be used in which(F_(min))*(MDDR)<(TDDR)^(−1/2) throughout the stretch, F_(max) is 2,F_(min) is 0.9, and the film is stretched to a TDDR of at least 4.6 andoften at least 6.8. If F_(max) is 1.5, then the film is stretched to aTDDR of at least 4.2 and often at least 6.1, If F_(max) is 1.2, then thefilm is stretched to a TDDR of at least 3.7 and often at least 5.4. Ifno limit is placed on F, then the film is stretched to a TDDR of atleast 8.4. A boundary trajectory can also be used in which(F_(min))*(MDDR)<(TDDR)^(−1/2) throughout the stretch, F_(max) is 1.5,F_(min) is 0.9, and the film is stretched to a TDDR of at least 6.8.

[0095] Other useful trajectories can be defined using F_(max). Usefultrajectories include coplanar trajectories where TDDR is at least 5, Uis at least 0.85 over a final portion of the stretch after achieving aTDDR of 2.5, and F_(max) is 2 during stretching. Useful trajectoriesalso include coplanar trajectories where TDDR is at least 6, U is atleast 0.7 over a final portion of the stretch after achieving a TDDR of2.5, and F_(max) is 2 during stretching.

[0096] Yet other useful coplanar trajectories include those in whichMDDR<TDDR^(−1/2)<(F_(max))*(MDDR) during a final portion of the draw inwhich TDDR is greater than a critical value TDDR′. The followingprovides minimum draw ratios that should be achieved for the trajectory.When TDDR′ is 2 or less, then for F_(max)=2, the minimum draw is 3.5;for F_(max)=1.5, the minimum draw is 3.2; and for F_(max)=2, the minimumdraw is 2.7. When TDDR′ is 4 or less, then for F_(max)=2, the minimumdraw is 5.8; for F_(max)=1.5, the minimum draw is 5.3; and forF_(max)=1.2, the minimum draw is 4.8. When TDDR′ is 5 or less, then forF_(max)=2, the minimum draw is 7; for F_(max)=1.5, the minimum draw is6.4; and for F_(max)=1.2, the minimum draw is 5.8.

[0097] In general, a variety of acceptable trajectories can beconstructed using curvilinear and linear tracks so that the overfeedremains below a critical maximum level throughout the drawing to preventfold-over defects while remaining above a critical minimum level toallow the desired level of truly uniaxial character with its resultingproperties.

[0098] A variety of sub-uniaxial and super-uniaxial trajectories may beformed using the parabolic shape. FIG. 18 illustrates examples thatdemonstrate a different levels minimum U after a critical TDDR and thatdemonstrate a different maximum overfeeds up to a final desired TDDR.The curves are represented by coordinates x and y as scaled by x₁, halfthe initial separation distance of the tracks. The scaled x coordinate,the quantity (x/x₁), is therefore equal to the TDDR. Curve 300 is theideal case with a value of x₁/x₀ of 1.0. Curve 302 is the parabolic casewith a value of x₁/x₀ of 0.653 in which U remains greater than 0.70above a draw ratio of 2.5. Curve 304 is the parabolic case with a valueof x₁/x₀ of 0.822 in which U remains above 0.85 after a draw ratio of2.5. Curves 306, 308, and 310 illustrate various levels of overfeed. Theoverfeed, TDDR and scaled inlet width are related by

x ₁ /x ₀=(F ²(TDDR)−1)/(TDDR−1)

[0099] It follows directly that the overfeed increases with increasingTDDR in the parabolic trajectories described here. Curve 306 is theparabolic case with a value of x₁/x₀ of 1.52 in which the overfeedremains below 1.2 up to a final draw ratio of 6.5. Curve 308 is theparabolic case with a value of x₁/x₀ of 2.477 in which the overfeedremains below 1.5 up to a final draw ratio of 6.5. Curve 310 is theparabolic case with a value of x₁/x₀ of 4.545 in which the overfeedremains below 2 up to a final draw ratio of 6.5. The level of overfeedis a function of the final draw ratio in these cases. For example, usinga value of x₁/x₀ of only 4.333 rather than 4.545 allows drawing to afinal TDDR of 10 while keeping the overfeed under 2.

[0100] For the parabolic trajectories, a relationship allows the directcalculation of MDDR at any given TDDR for a fixed scaled inlet width:

MDDR=(TDDR(x ₁ /x ₀)+(1−x ₁ /x ₀))^(−1/2)

[0101] One observation is that the relationship between MDDR and TDDR isnot an explicit function of the y position. This allows the constructionof composite hybrid curves comprising sections of parabolic trajectoriesthat are vertically shifted in y/x₁. FIG. 19 illustrates one method. Aparabolic trajectory for the initial portion of the draw is chosen,curve 320 and a parabolic trajectory is chosen for the final portion,curve 322. The initial curve 320 is chosen to provide a super-uniaxialdraw with a maximum overfeed of 2.0 at a draw ratio of 4.5. Curve 320has a scaled inlet width of 4.857. The final curve 322 is chosen to be asub-uniaxial draw with a minimum U of 0.9 at the 4.5 draw ratio. Curve322 has a scaled inlet width of 0.868. The actual track or rail shapefollows curve 320 up to TDDR of 4.5 and then continues on curve 324which is a vertically shifted version of curve 322. In other words, atrajectory can have an initial stretching zone with tracks having afunctional form correspond to:

±(x)/(x ₁)=(1/4)(x ₁ /x ₀)(y/x ₁)²+1

[0102] and then a later stretching zone with tracks having a functionalform corresponding to

±(x)/(x ₂)=(1/4)(x ₂ /x ₀)((y−A)/x ₂)²+1;

[0103] where x₁ and x₂ are different and A corresponds to the verticalshift that permits coupling of the trajectories. Any number of parabolicsegments may be combined in this manner.

[0104] The parabolic trajectories, and their composite hybrids, can beused to guide the construction of related trajectories. One embodimentinvolves the use of linear segments to create trajectories. These linearapproximations can be constructed within the confines of parabolictrajectories (or composite hybrids) of maximum overfeed and minimumoverfeed (or minimum U) at a chosen TDDR′ larger than a critical drawratio, TDDR*. Values for TDDR* can be selected which relate to the onsetof strain-induced crystallinity with examples of values of 1.5, 2, and2.5 or may be related to elastic strain yielding with lower values of1.2 or even 1.1. The range of TDDR* generally falls between 1.05 and 3.Portions of the rail or track below TDDR* may not have any particularconstraints on minimum overfeed or U and may fall outside the confinesof the constraining parabolic trajectories. In FIG. 20, curve 340 ischosen to be the constraining parabolic trajectory of minimum overfeedat the chosen draw ratio, TDDR′, illustrated here at a value of 6.5. Forillustration, the minimum overfeed constraining parabolic trajectory hasbeen chosen as the ideal curve with a scaled inlet width of unity. Usingthe relationship between overfeed, TDDR and scaled inlet width, curve342 is identified as the constraining parabolic trajectory of maximumoverfeed where the maximum value of F is 2.0 at the TDDR value of 6.5.Curve 342 is now vertically shifted to form curve 344 so that the twoconstraining parabolic trajectories meet at the chosen TDDR′ of 6.5. Itshould be remarked that curves 342 and 344 are completely equivalentwith respect to drawing character. Curve 344 merely delays the stretchuntil a later spatial value of y/x₁ of 2.489. An approximation of linearor non-parabolic curvilinear segments will tend to lie between theseconstraining trajectories above TDDR*.

[0105] Unlike parabolic trajectories that possess increasing divergenceangles with increasing TDDR, linear trajectories have a fixed divergenceangle. Thus the overfeed decreases with increasing TDDR along a linearsegment. A simple linear approximation can be constructed by choosing aline with a divergence angle equal to the desired minimum overfeed atthe chosen TDDR. The line segment may be extrapolated backwards in TDDRuntil the overfeed equals the maximum allowed. A subsequent linearsegment is started in similar fashion. The procedure is repeated asoften as necessary or desired. As the maximum overfeed decreases, thenumber of segments needed for the approximation increases. When the TDDRdrops below TDDR*, any number of methods may be used to complete thetrack or rail as long as the constraint on maximum overfeed ismaintained. In FIG. 20, curve 346 is a linear approximation constrainedby a maximum overfeed of 2. Because of this large maximum overfeed, itcomprises only two linear sections. The final linear segment extends allthe way backwards from the chosen TDDR of 6.5 to a lower TDDR of 1.65.In this case, TDDR* is taken as 2. Without a constraint on U below aTDDR of 2, one method of finishing the track is to extrapolate a secondlinear segment from TDDR at 1.65 back to TDDR of unity at the y/x₁ zeropoint. Note that this causes the second segment to cross the lowerconstraining parabola, since the constraint is not effective belowTDDR*.

[0106] In FIG. 20, curve 348 is the result of using a tighter value forthe maximum overfeed of 1.5. Here the constraining parabolic trajectoryof maximum overfeed is not shown. Three linear segments are required.The first segment extends backwards from TDDR of 6.5 to TDDR of 2.9. Thesecond segment assumes a divergence angle equal to the constrainingparabolic trajectory of minimum overfeed at this TDDR value of 2.9 andextends backwards to a TDDR of 1.3. This second segment ends belowTDDR*. The final segment completes the track or rail shape for curve 348using a different method than that used for curve 346. Here the sameprocedure for the last segment is used as for the previous segments,resulting in a delay of the onset of stretching with higher y/x₁ valueof. A third method of completing the track is to set the overfeed to themaximum at the initial TDDR of unity.

[0107] General, non-linear and non-parabolic trajectories fitting therequirements of the present invention can be constructed using theconstraining parabolic trajectories. The maximum overfeed constrainingparabolic trajectory is the curve of minimum slope, i.e. maximumdivergence angle, as a function of TDDR. The minimum overfeedconstraining parabolic trajectory is the curve of maximum slope, i.e.minimum divergence angle, as a function of TDDR. In general, curves canbe extrapolated backwards from the chosen TDDR′ using any function ofslope that lies between the constraining bounds. A simple method fordefining a function for the slope that lies between these constraints isto take a simple linear combination of known curves within the envelope.Curve 350 in FIG. 20 illustrates this simple method. In this example,350 is formed by a linear combination of the maximum overfeedconstraining parabolic trajectory, curve 344, and the linearapproximation to it, curve 346, with the linear weights of 0.7 and 0.3,respectively. In general, functions that are not simple linearcombinations can also be used.

[0108] The aforementioned method for describing the variousnon-parabolic trajectories of the present invention can be applied overdifferent sections of the track, e.g. the example of FIG. 20 for TDDR upto 6.5 may be combined with another section for TDDR over 6.5 withdifferent requirements and therefore different maximum and minimumconstraining trajectories over that higher range of TDDR. In this case,the TDDR′ of the previous section of lower draw takes on the role ofTDDR*. In general, TDDR′ may be chosen across the range of desireddrawing. Various sections may be used to account for the variousphenomenon of drawing, such as yielding, strain-induced crystallization,onset of necking or other draw non-uniformity, onset of strain-hardeningor to account for the development of various properties within the film.Typical break points include those for TDDR*, the range of 3 to 7 forstrain-hardening in polyesters, and typical final draw values in therange of 4 to 10 or more.

[0109] The procedures for determining boundary trajectories for thepresent invention in the method of extrapolating backwards to lower TDDRfrom a chosen TDDR′ may be used in an analogous method of extrapolatingforward to higher TDDR from a chosen TDDR″. Again, two constrainingtrajectories are formed, joined at the lowest chosen TDDR″. A convenientvalue for TDDR″ is the initial TDDR of unity. In this method, theconstraining trajectory of minimum overfeed or U lies above the maximumoverfeed curve. FIG. 19 actually exhibits an example of this method inwhich the hybrid curve 324 lies between the minimum overfeed constraint,curve 322, and the maximum overfeed constraint, curve 320.

[0110] Still another class of boundary trajectories can be defined andmay, in some embodiments, be useful in suppressing residual wrinkles.Because the uniaxial condition in the absence of shear provides aprincipal MD stress of zero, it is anticipated, using finite strainanalysis, that the principal MD stress will actually go into slightcompression under these conditions. Using finite strain analysis and aNeo-Hookean elastic solid constitutive equation, it is discovered that asuitable criterion for preventing compressive stresses may optionally begiven by the following equation:

((TDDR)(MDDR))⁻⁴+((TDDR)(MDDR))²−(TDDR)⁻²−(MDDR)⁻²−sin²(θ)((TDDR)(MDDR))⁻²=0

[0111] where MDDR is the cosine of the divergence angle. This optionalmethod of the present invention then specifies this class of boundarytrajectories.

[0112] As indicated above, the film may be drawn out-of-plane usingout-of-plane boundary trajectories, i.e. boundary trajectories that donot lie in a single Euclidean plane. There are innumerable, butnevertheless particular, boundary trajectories meeting relationalrequirements of this preferred embodiment of the present invention, sothat a substantially uniaxial draw history may be maintained usingout-of-plane boundary trajectories. The boundaries may be symmetrical,forming mirror images through a central plane, e.g. a plane comprisingthe initial center point between the boundary trajectories, the initialdirection of film travel and the initial normal to the unstretched filmsurface. In this embodiment, the film may be drawn between the boundarytrajectories along a cylindrical space manifold formed by the set ofline segments of shortest distance between the two opposing boundarytrajectories as one travels along these boundary trajectories at equalrates of speed from similar initial positions, i.e., colinear with eachother and the initial center point. The trace of this ideal manifold onthe central plane thus traces out the path of the film center for anideal draw. The ratio of the distance along this manifold from theboundary trajectory to this central trace on the central plane to theoriginal distance from the start of the boundary trajectory to theinitial center point is the instantaneous nominal TDDR across the filmspanning the boundary trajectories, i.e. the ratios of thehalf-distances between the current opposing points on the boundarytrajectories and the half-distances between the initial positions of theopposing points on the boundary trajectories. As two opposing pointsmove at constant and identical speeds along the opposing boundarytrajectories, the corresponding center point on the central tracechanges speed as measured along the arc of the central trace, i.e. thecurvilinear MD. In particular, the central trace changes in proportionwith the projection of the unit tangent of the boundary trajectory onthe unit tangent of the central trace.

[0113] The classes of trajectories described above are illustrative andshould not be construed as limiting. A host of trajectory classes areconsidered to lie within the scope of the present invention. Asindicated above, the primary stretching region can contain two or moredifferent zones with different stretching conditions. For example, onetrajectory from a first class of trajectories can be selected for aninitial stretching zone and another trajectory from the same first classof trajectories or from a different class of trajectories can beselected for each of the subsequent stretching zones.

[0114] The present invention encompasses all nearly uniaxial boundarytrajectories comprising a minimum value of U of about 0.7, morepreferably approximately 0.75, still more preferably about 0.8 and evenmore preferably about 0.85. The minimum U constraint may be applied overa final portion of the draw defined by a critical TDDR preferably ofabout 2.5, still more preferably about 2.0 and more preferably about1.5. In some embodiments, the critical TDDR can be 4 or 5. Above acritical TDDR, certain materials, e.g. certain monolithic and multilayerfilms comprising orientable and birefringent polyesters, may begin tolose their elasticity or capability of snap back because of thedevelopment of structure such as strain-induced crystallinity. Thecritical TDDR may coincide with a variety of material and process (e.g.temperature and strain rate) specific events such as the critical TDDRfor the onset of strain-induced crystallization. The minimum value of Uabove such a critical TDDR can relate to an amount of non-uniaxialcharacter set into the final film.

[0115] A variety of boundary trajectories are available when U issubuniaxial at the end of the stretching period. In particular, usefulboundary trajectories include coplanar trajectories where TDDR is atleast 5, U is at least 0.7 over a final portion of the stretch afterachieving a TDDR of 2.5, and U is less than 1 at the end of the stretch.Other useful trajectories include coplanar and non-coplanar trajectorieswhere TDDR is at least 7, U is at least 0.7 over a final portion of thestretch after achieving a TDDR of 2.5, and U is less than 1 at the endof the stretch. Useful trajectories also include coplanar andnon-coplanar trajectories where TDDR is at least 6.5, U is at least 0.8over a final portion of the stretch after achieving a TDDR of 2.5, and Uis less than 1 at the end of the stretch. Useful trajectories includecoplanar and non-coplanar trajectories where TDDR is at least 6, U is atleast 0.9 over a final portion of the stretch after achieving a TDDR of2.5, and U is less than 1 at the end of the stretch.

[0116] Useful trajectories also include coplanar and non-coplanartrajectories where TDDR is at least 7 and U is at least 0.85 over afinal portion of the stretch after achieving a TDDR of 2.5.

[0117] In some embodiments, a small level of MD tension is introducedinto the stretching process to suppress wrinkling. Generally, althoughnot necessarily, the amount of such MD tension increases with decreasingU.

[0118] In some embodiments, it is useful to increase the tension as thedraw proceeds. For example, a smaller value of U earlier in the draw maytend to set more non-uniaxial character into the final film. Thus it maybe advantageous to combine the attributes of various trajectory classesinto composite trajectories. For example, a uniaxial parabolictrajectory may be preferred in the earlier portions of the draw, whilethe later portions of the draw may converge on a different trajectory.In another arrangement, U may be taken as a non-increasing function withTDDR. In still another arrangement, the overfeed, F, may be anon-increasing function with TDDR after a critical draw ratio of, forexample, 1.5, 2, or 2.5.

[0119] The uniaxial parabolic trajectory assumes a uniform spatialdrawing of the film. Good spatial uniformity of the film can be achievedwith many polymer systems with careful control of the crossweb anddownweb caliper (thickness) distribution of the initial, undrawn film orweb, coupled with the careful control of the temperature distribution atthe start of and during the draw. For example, a uniform temperaturedistribution across the film initially and during draw on a film ofinitially uniform caliper should suffice in most cases. Many polymersystems are particularly sensitive to non-uniformities and will draw ina non-uniform fashion if caliper and temperature uniformity areinadequate. For example, polypropylenes tend to “line draw” undermono-axial drawing. Certain polyesters, notably polyethylenenaphthalate, are also very sensitive.

[0120] Non-uniform film stretching can occur for a variety of reasonsincluding, for example, non-uniform film thickness or other properties,non-uniform heating, etc. In many of these instances, portions of thefilm near the gripping members draws faster than that in the center.This creates an MD tension in the film that can limit ability to achievea final uniform MDDR. One compensation for this problem is to modify theparabolic or other uniaxial trajectory to present a lower MDDR. In otherwords, MDDR<(TDDR)^(−1/2) for a portion or all of the draw.

[0121] In one embodiment, a modified parabolic or other uniaxialtrajectory is selected in which MDDR<(TDDR)^(−1/2), corresponding to alarger divergence angle, for all of the draw. In at least someinstances, this condition can be relaxed because a U value of less thanunity is acceptable for the application. In such instances, a modifiedparabolic or other uniaxial trajectory is selected in which(0.9)MDDR<(TDDR)^(−1/2).

[0122] In another embodiment, a modified parabolic or other uniaxialtrajectory is selected in which MDDR<(TDDR)^(−1/2) for an initialstretching zone in which the TDDR is increase by at least 0.5 or 1. Adifferent trajectory is then maintained for the remainder of the draw.For example, a later stretching zone (within the stretching region 34)would have a parabolic or other uniaxial trajectory in which MDDR isequal to or approximately equal to (within ±5% and, preferably, within±3%)(TDDR)^(−1/2). As an example, the initial stretching zone canaccomplish a TDDR level up to a desired value. This desired value istypically no more than 4 or 5. The later stretching zone can thenincrease the TDDR from the desired value of the initial stretching zone(or from a higher value if there are intervening stretching zones).Generally, the later stretching zone is selected to increase the TDDRvalue by 0.5 or 1 or more.

[0123] Again, in at least some instances, the MDDR and TDDR relationshipcan be relaxed because a U value of less than unity is acceptable forthe application. In such instances, the modified parabolic or otheruniaxial trajectory of the initial stretching zone is selected in which(0.9)MDDR<(TDDR)^(−1/2).

[0124] Returning to FIG. 5, the apparatus typically includes apost-conditioning region 36. For example, the film may be set in zone 48and quenched in zone 50. In some embodiments, quenching is performedoutside the stretching apparatus. Typically, the film is set when atleast one component of the film, e.g. one layer type in a multilayerfilm, reaches a temperature below the glass transition. The film isquenched when all components reach a temperature level below their glasstransitions. In the embodiment illustrated in FIG. 5, a takeaway systemis used to remove the film from the primary stretching region 34. In theillustrated embodiment, this takeaway system is independent of (i.e.,not directly connected to) the tracks upon which the film was conveyedthrough the primary stretching region. The takeaway system can use anyfilm conveyance structures such as tracks 140, 141 with gripping memberssuch as, for example, opposing sets of belts or tenter clips.

[0125] In some embodiments as illustrated in FIG. 10, TD shrinkagecontrol can be accomplished using tracks 140′, 141′ which are angled (ascompared to parallel tracks 140, 141 that could be used in otherembodiments of a suitable take-away system). For example, the tracks ofthe take-away system can be positioned to follow a slowly convergingpath (making an angle θ of no more than about 5°) through at least aportion of the post conditioning region to allow for TD shrinkage of thefilm with cooling. The tracks in this configuration allow the control ofTD shrinkage to increase uniformity in the shrinkage. In otherembodiments, the two opposing tracks can be diverging typically at anangle of no more than about 3° although wider angles can be used in someembodiments. This can be useful to increase the MD tension of the filmin the primary stretching region to, for example, reduce propertynon-uniformity such as the variation of principal axes of refractiveindex across the film.

[0126] In some embodiments, the position of the take-away system can beadjustable to vary the position along the stretching apparatus at whichthe take-away system grips the film, as illustrated in FIG. 11. Thisadjustability provides one way to control the amount of stretching towhich the film is subjected. Film received by tracks 140′, 141′ of atake-away system earlier in the draw (shown by dotted lines in FIG. 11)will generally have a smaller TDDR than would film received by a tracks140, 141 of a take-away system positioned later in the draw (shown insolid lines in FIG. 11). The take-away system can also, optionally, beconfigured to allow adjustment in the distance between the opposingtracks of the take-away system. In addition, the take-away system canalso, optionally, be configured to allow adjustment in the length of thetake-away system.

[0127] Another example of a possible take-away system includes at leasttwo different regions with separated tracks 140, 141, 142, 143. Theseregions can be formed using two separate sets 140, 141 and 142, 143 ofopposing tracks as illustrated in FIG. 12. In one embodiment,illustrated in FIG. 12, the first region can include tracks 140, 141that are disposed at a convergence angle to provide TD shrinkage controland the tracks 142, 143 in the second regions can be parallel. In otherembodiments, the opposing tracks of the two different regions can be setat two different convergence angles to provide TD shrinkage control, asdescribed above, or the first region can have parallel tracks and thesecond region have tracks disposed at a convergence angle to provide TDshrinkage control. Alternatively or additionally, the two differenttracks can be set at two different takeaway speeds to decouple theprimary stretching region from a takeaway region that applies tension toremove wrinkles.

[0128] In one embodiment a the take-away system illustrated in FIG. 12,the tracks 142′, 143′ are nested within the opposing tracks 140, 141prior to receiving the film. When the film is initially received by theopposing tracks 140, 141, the tracks 142, 143 move to the positionillustrated in FIG. 12. In other embodiments, the opposing tracks 140,141, 142, 143 are positioned as illustrated in FIG. 12 (i.e., notnested) in the absence of any film.

[0129] Another example of a take-away system is illustrated in FIG. 13.In this example, the tracks 140, 141 of the take-away system are angledwith respect to the centerline of the film as the film is conveyedthrough the tracks 64 of the primary stretching region. The angle of thetwo opposing conveyance mechanisms can be the same, for example, anangle β or the angle can be different and can be described as β+ε forone track and β−ε for the other track. Typically, P is at least 1° andcan be an angle of 5°, 10°, or 20° degrees or more. The angle ε wouldcorrespond to the converging or embodiments, the tracks 64 in theprimary stretching zone can also be disposed at an angle φ and thetracks 140, 141 are angled at φ+β+ε and φ+β−ε as illustrated in FIG. 13.An angled take-away system, primary stretching zone, or both can beuseful to provide films where the principal axis or axes of an propertyof the film, such as the refractive index axes or tear axis, is angledwith respect to the film. In some embodiments, the angle that thetake-away system makes with respect to the primary stretching zone isadjustable manually or mechanically using a computer-controlled driveror other control mechanism or both.

[0130] In some embodiments using an angled take-away system, the twoopposing tracks are positioned to receive film having the same orsubstantially similar TDDR (where the dotted line indicates film at thesame TDDR), as illustrated in FIG. 13. In other embodiments, the twoopposing tracks 140, 141 are positioned to receive the film so that theTDDR is different for the two opposing tracks (the dotted line of FIG.14 indicates film at the same TDDR), as illustrated in FIG. 14. Thislatter configuration can provide a film with properties that change overthe TD dimension of the film.

[0131] Typically, the portions of the film that were held by thegripping members through the primary stretching region are removed. Tomaintain a substantially uniaxial draw throughout substantially all ofthe draw history (as shown in FIG. 5), at the end of the transversestretch, the rapidly diverging edge portions 56 are preferably severedfrom the stretched film 48 at a slitting point 58. A cut can be made at58 and flash or unusable portions 56 can be discarded.

[0132] Release of the selvages from a continuous gripping mechanism canbe done continuously; however, release from discrete grippingmechanisms, such as tenter clips, should preferably be done so that allthe material under any given clip is released at once. This discreterelease mechanism may cause larger upsets in stress that may be felt bythe drawing web upstream. In order to assist the action of the isolatingtake-away device, it is preferred to use a continuous selvage separationmechanism in the device, e.g. the “hot” slitting of the selvage from thecentral portion of a heated, drawn film.

[0133] The slitting location is preferably located near enough to the“gripline”, e.g. the isolating takeaway point of first effective contactby the gripping members of the take-away system, to minimize or reducestress upsets upstream of that point. If the film is slit before thefilm is gripped by the take-away system, instable takeaway can result,for example, by film “snapback” along TD. The film is thus preferablyslit at or downstream of the gripline. Slitting is a fracture processand, as such, typically has a small but natural variation in spatiallocation. Thus it may be preferred to slit slightly downstream of thegripline to prevent any temporal variations in slitting from occurringupstream of the gripline. If the film is slit substantially downstreamfrom the gripline, the film between the takeaway and boundary trajectorywill continue to stretch along TD. Since only this portion of the filmis now drawing, it now draws at an amplified draw ratio relative to theboundary trajectory, creating further stress upsets that could propagateupstream, for example, undesirable levels of machine direction tensionpropagating upstream.

[0134] The slitting is preferably mobile and re-positionable so that itcan vary with the changes in takeaway positions needed to accommodatevariable final transverse draw direction ratio or adjustment of theposition of the take-away system. An advantage of this type of slittingsystem is that the draw ratio can be adjusted while maintaining the drawprofile simply by moving the take-away slitting point 58.

[0135] A variety of slitting techniques can be used including a heatrazor, a hot wire, a laser, a focused beam of intense IR radiation or afocused jet of heated air. In the case of the heated jet of air, the airmay be sufficiently hotter in the jet to blow a hole in the film, e.g.by heat softening, melting and controlled fracture under the jet.Alternatively, the heated jet may merely soften a focused section of thefilm sufficiently to localize further drawing imposed by the stilldiverging boundary trajectories, thus causing eventual fracturedownstream along this heated line through the action of continued filmextension. The focused jet approach may be preferred in some cases,especially when the exhaust air can be actively removed, e.g. by avacuum exhaust, in a controlled fashion to prevent stray temperaturecurrents from upsetting the uniformity of the drawing process. Forexample, a concentric exhaust ring around the jet nozzle can be used.Alternatively, an exhaust underneath the jet, e.g. on the other side ofthe film, can be used. The exhaust may be further offset or supplementeddownstream to further reduce stray flows upstream into the drawing zone.

[0136] Another attribute of the take-away system is a method of speedand or MD tension control so that the film can be removed in a mannercompatible with the output speed. This take-away system could also beused to pull out any residual wrinkles in the film. The wrinkles couldbe initially pulled out during start up by a temporary increase in thetakeaway speed above the output speed of the final, released portion ofthe drawn film, or the wrinkles could be pulled out by a constant speedabove the output film MD speed during continuous operation, e.g. in thecase of a super-uniaxial draw in the final portion of draw. The speed ofthe takeaway can also be set above the MD velocity of the film along theboundary trajectories at the gripline. This can be used to alter theproperties of the film. This over-speed of the takeaway can also reducethe final value of U and is thereby limited by this consideration in thecontext of the final end use of the film.

[0137] The process also includes a removal portion in region 38.Optionally a roller 65 may be used to advance the film, but this may beeliminated. Preferably the roller 65 is not used as it would contact thestretched film 52 with the attendant potential to damage the stretchedfilm. Another cut 60 may be made and unused portion 61 may be discarded.Film leaving the take-away system is typically wound on rolls for lateruse. Alternatively, direct converting may take place after take away.

[0138] The principles of MD and TD shrinkage control described above canalso be applied to other stretching apparatuses including theconventional tenter configuration illustrated in FIG. 1. FIG. 17illustrated an embodiment in which the tracks 64 from a primarystretching region (such as the linear diverging tracks illustrated inFIG. 1) continue into or through a portion of a post-conditioningregion. The film is then optionally captured by an isolated takeawaysystem 140, 141, if desired. The continuation of the tracks 64 can beused to cool the film and allow for shrinkage of the film. In someembodiments, the continued tracks 164 follow a slowly converging path(making an angle θ of no more than about 5°) through at least a portionof the post conditioning region to allow for TD shrinkage of the filmwith cooling. The tracks in this configuration allow the control of TDshrinkage to increase uniformity in the shrinkage. In some embodiments,the tracks 264 follow a more aggressively converging path (making anangle φ of at least 15°, and typically in the range of 20° and 30°)through at least a portion of the post conditioning region to provide MDshrinkage control of the film with cooling. In some embodiments asillustrated in FIG. 17, the post conditioning region includes bothslowly converging tracks 164 and more aggressively converging tracks264. In other embodiments, only one set of tracks 164 and tracks 264 isused.

[0139] The present invention should not be considered limited to theparticular examples described above, but rather should be understood tocover all aspects of the invention as fairly set out in the attachedclaims. Various modifications, equivalent processes, as well as numerousstructures to which the present invention may be applicable will bereadily apparent to those of skill in the art to which the presentinvention is directed upon review of the instant specification.

What is claimed is:
 1. An apparatus for processing a film, the apparatuscomprising: a conveyor configured and arranged to convey the film alonga machine direction within the apparatus, the conveyor comprisinggripping members that are configured and arranged to hold opposing edgeportions of the film, a portion of the conveyor is configured andarranged to provide diverging paths along which the gripping membersmove to stretch the film; and an isolated take-away system that receivesthe film from the conveyor after stretching the film, the take-awaysystem comprising opposing tracks and gripping members configured andarranged to grasp opposing take-away regions of the film after a desiredamount of stretching and convey the opposing take-away regions of thefilm along the opposing tracks, wherein the opposing tracks define aregion in which at least a portion of the opposing tracks are angledtoward each other.
 2. The apparatus of claim 1, wherein the opposingtracks are angled toward each other along an entire length of theopposing tracks.
 3. The apparatus of claim 1, wherein each of theopposing tracks comprises at least a first track section and a secondtrack section, wherein the first track sections of the opposing tracksare angled toward each other.
 4. The apparatus of claim 1, wherein eachof the opposing tracks comprises at least a first track section and asecond track section, wherein the second track sections of the opposingtracks are angled toward each other.
 5. The apparatus of claim 1,wherein the portions of the opposing tracks are each angled toward eachother at an angle of no more than 3° relative to the direction alongwhich the film is conveyed.
 6. The apparatus of claim 1, whereintake-away system is disposed at an angle relative to the direction alongwhich the film is conveyed.
 7. A method of processing a film, the methodcomprising: holding opposing edge portions of a film using grippingmembers; conveying the film along diverging paths and in a machinedirection within a stretching region of a stretching apparatus tostretch the film; receiving the film after stretching in an isolatedtake-away system by grasping opposing take-away regions of the filmsusing opposing gripping members of the take-away system; and conveyingthe film through a portion of the take-away system in which the opposinggripping members are angled toward each other.
 8. The method of claim 7,wherein the opposing gripping members are angled toward each other alongan entire length of the take-away system.
 9. The method of claim 7,wherein the take-away system comprises at least a first set of opposinggripping members and a second set of opposing gripping members, whereinthe first set of opposing gripping members are angled toward each other.10. The method of claim 7, wherein the take-away system comprises atleast a first set of opposing gripping members and a second set ofopposing gripping members, wherein the second set of opposing grippingmembers are angled toward each other.
 11. The method of claim 7, whereinthe portion of the opposing gripping members are angled toward eachother at an angle of no more than 3° relative to the direction alongwhich the film is conveyed.
 12. An apparatus for processing a film, theapparatus comprising: a conveyor comprising gripping members that holdopposing edge portions of the film and convey, under influence of adrive member, the film along a machine direction within the apparatus,wherein a portion of the conveyor is configured and arranged to providediverging paths along which the gripping members move to stretch thefilm; and an isolated take-away system that receives the film from theconveyor after stretching the film, the take-away system comprising afirst set of opposing tracks, a second set of opposing tracks, aplurality of first gripping members configured and arranged to graspopposing first take-away regions of the film after a desired amount ofstretching and convey the film along the first set opposing tracks, anda plurality of second gripping members configured and arranged to graspopposing second take-away regions of the film and convey the film alongthe second set of opposing tracks, wherein the second take-away regionsare nearer a center of the film than the first take-away regions. 13.The apparatus of claim 12, wherein the second gripping members areconfigured and arranged to move from a first position disposed at leastpartially between the first gripping members and a second position inwhich the second gripping members are not partially disposed between thefirst gripping members.
 14. The apparatus of claim 12, wherein the firstgripping members are angled toward each other.
 15. The apparatus ofclaim 12, wherein the second gripping members are angled toward eachother.
 16. An apparatus for processing a film, the apparatus comprising:a conveyor comprising gripping members that hold opposing edge portionsof the film and convey, under influence of a drive member, the filmalong a machine direction within the apparatus, wherein a portion of theconveyor is configured and arranged to provide diverging paths alongwhich the gripping members move to stretch the film; and an isolatedtake-away system that receives the film from the conveyor afterstretching the film, the take-away system comprising opposing tracks anda plurality of gripping members configured and arranged to graspopposing take-away regions of the film after a desired amount ofstretching and convey the opposing take-away regions of the film alongthe opposing tracks, wherein the opposing tracks are disposed at anangle of at least 1° with respect to the machine direction.
 17. Theapparatus of claim 16, wherein the at least a portion of the opposingtracks of the take-away system are angled toward each other.
 18. Theapparatus of claim 16, wherein the opposing tracks are disposed atdifferent angles with respect to the machine direction.
 19. Theapparatus of claim 16, wherein the opposing tracks of the take-awaysystem are configured and arranged to grasp the opposing take-awayregions of the film at a position in which the opposing take-awayregions of the film have a same transverse directional draw ratio. 20.The apparatus of claim 16, wherein the opposing tracks of the take-awaysystem are configured and arranged to grasp the opposing take-awayregions of the film at a position in which the opposing take-awayregions of the film have different transverse directional draw ratios.21. The apparatus of claim 16, wherein the diverging paths areconfigured and arranged to stretch the film so that a centerline of thefilm is angled with respect to a machine direction of the conveyor priorto stretching the film.
 22. An apparatus for processing a film, theapparatus comprising: a conveyor comprising gripping members that holdopposing edge portions of the film and convey, under influence of adrive member, the film along a machine direction within the apparatus,wherein a portion of the conveyor is configured and arranged to providediverging paths along which the gripping members move to stretch thefilm; and an isolated take-away system that receives the film from theconveyor after stretching the film, the take-away system comprisingopposing tracks and a plurality of gripping members configured andarranged to grasp opposing take-away regions of the film after a desiredamount of stretching and convey the opposing take-away regions of thefilm along the opposing tracks, wherein the apparatus is configured andarranged to allow selection of a final transverse direction draw ratioof the film by changing a position of the isolated take-away system withrespect to a position of the conveyor.
 23. An apparatus for processing afilm, the apparatus comprising: a conveyor configured and arranged toconvey the film along a machine direction within the apparatus, theconveyor comprising gripping members that are configured and arranged tohold opposing edge portions of the film, a portion of the conveyor isconfigured and arranged to provide diverging paths along which thegripping members move to stretch the film; and an isolated take-awaysystem that receives the film from the conveyor after stretching thefilm, the take-away system comprising opposing tracks and grippingmembers configured and arranged to grasp opposing take-away regions ofthe film after a desired amount of stretching and convey the opposingtake-away regions of the film along the opposing tracks, wherein theopposing tracks define a region in which at least a portion of theopposing tracks are angled away from each other.
 24. The apparatus ofclaim 23, wherein the opposing tracks are angled away from each otheralong an entire length of the opposing tracks.
 25. The apparatus ofclaim 23, wherein each of the opposing tracks comprises at least a firsttrack section and a second track section, wherein the first tracksections of the opposing tracks are angled away from each other.
 26. Theapparatus of claim 23, wherein each of the opposing tracks comprises atleast a first track section and a second track section, wherein thesecond track sections of the opposing tracks are angled away from eachother.
 27. The apparatus of claim 23, wherein the portions of theopposing tracks are each angled away from each other at an angle of nomore than 3° relative to the direction along which the film is conveyed.28. An apparatus for processing a film, the apparatus comprising: aconveyor configured and arranged to convey the film along a machinedirection, the conveyor comprising gripping members that are configuredand arranged to hold opposing edge portions of the film; a stretchingregion in which the gripping members are configured arranged to travelalong diverging paths to stretch the film; and a post-conditioningregion disposed after the stretching region and comprising at least onezone in which the gripping members are configured and arranged to travelalong converging paths.
 29. The apparatus of claim 28, wherein thepost-conditioning region comprises at least a first zone and a secondzone, wherein the gripping members are configured and arranged to travelalong paths converging at a first angle in the first zone and alongpaths converging at a second angle in the second zone, wherein the firstand second angles are substantially different.
 30. The apparatus ofclaim 29, wherein the first angle is no more than about 3° and thesecond angle is at least about 15°.
 31. The apparatus of claim 28,wherein the converging paths in at least one zone of thepost-conditioning region converge at an angle of no more than 3°. 32.The apparatus of claim 28, wherein the converging paths in at least onezone of the post-conditioning region converge at an angle of at least15°.
 33. The apparatus of claim 28, wherein the converging paths in atleast one zone of the post-conditioning region converge at an angle ofin the range of 20° to 30°.